In a conventional engine, the accessory housing fitted with fuel pumps, oil pumps for lubricating bearings, hydraulic pumps for controlling various members, electricity generators, and the starter, is placed outside the engine and receives power taken from the engine by means of a vertical shaft and angle gearing.
As time moves on, increasing compression ratios and turbine inlet temperatures, and also improvements in materials and in efficiency have led to the size of engines being constantly reduced so as to obtain ever greater thrust/weight ratios, and this applies both to civilian applications and to military applications.
The power takeoff system and the accessory housing have not been able to keep up with this progress and therefore represents an ever increasing proportion of the volume and the mass of engines, particularly low power engines that are of small size, and above all when the accessory housing, which is generally placed beneath or on the engine, and sometimes to one side of it, also contains an air starter and an electricity generator that are separate from each other.
The use of small engines, of ever-increasing simplicity and reduced cost, for propelling trainer airplanes, observation or attack drones, and cruise missiles, is requiring engine manufacturers to make these engines ever more furtive. This can only be achieved by greatly reducing the frontal area of the engine, which also achieves a significant reduction in drag that enables the range or duration of remote controlled aircraft fitted with such engines to be increased. In order to reduce the weight and the frontal area of such engines, it therefore appears desirable to consider integrating an electric generator/starter inside the engine and to eliminate the use of mechanical connections for the interface between the engine and the accessories, with said interface then being by way of electrical transmission.
On airliners, having electrical or electrohydraulic flight controls in ever increasing numbers, and also on radar, advance warning, or electronic surveillance airplanes, electricity requirements are large. The engines of such airplanes are fitted with auxiliary generators, thereby increasing the size of their auxiliary housings and also their weight. It is therefore advantageous to integrate an auxiliary generator in addition to the generator/starter in an engine having a high by-pass ratio so as to reduce the size and weight of angle takeoffs, or indeed to eliminate them, thus enabling the pod to be made narrower, by housing certain electrically-driven accessories in the pylon.
The state of the art is illustrated in particular by U.S. Pat. Nos. 3,859,785, 5,867,979, 3,264,482; and GB 1,147,730.
U.S. Pat. No. 3,859,785 relates to a single shaft turbomachine in which a bell fitted with permanent magnets forms the rotor of a generator and serves as a nut for the front bearing of the compressor shaft. The bell is disposed in the air inlet cone of the compressor. That solution presents the drawback of limiting the size of the electricity generator because of the small diameters of the shaft and the inlet cone. The compressor must be rotating at very high speed in order to deliver significant power from such a small unit.
U.S. Pat. No. 5,867,979 relates to a high by-pass ratio turbomachine having three shafts. In its FIG. 1, that document shows a main electricity generator disposed coaxially in the rear cone of the low pressure turbine, i.e. in a hot zone, and auxiliary generators disposed in the engine casing and driven by angle gearing and transverse shafts, the auxiliary generators receiving power respectively from the high pressure shaft and from the intermediate shaft. That document describes a second embodiment, shown in its FIGS. 2 and 3, in which the front and rear bearings of the shafts are of the electromagnetic type and have integral electricity generators. Because those generators are integrated on the shafts, they are small in size and they must rotate at high speed in order to deliver significant amounts of power from such a small volume. In addition, the generators integrated on the rear bearings are disposed in hot zones.
U.S. Pat. No. 3,264,482 shows a double-flow two-shaft turbomachine including a generator/starter interposed between a stationary disk and the support bearing for the compressor rotor on the drive shaft of the fan.
The field magnetic circuit is mounted at the periphery of a disk of the rotor surrounding a sleeve provided at the front of the compressor rotor, and the support via the inter-shaft bearing, and the secondary magnetic circuit is mounted inside the engine casing. The disk of the generator/starter rotor is bulky and leads to a considerable increase in weight.
GB 1 147 730 shows the state of the prior art closest to the invention, since its FIG. 3 shows an electric starter whose field magnetic circuit is mounted in the bore of a bladed disk 32 of a compressor rotor and surrounds the secondary magnetic circuit which is secured to the stator of the compressor, without excessive increase in weight, said starter also being capable of being used as an electricity generator.